The invention relates generally to means for indicating the status of one of a plurality of switches which are connected in series and, more particularly, to an indicating device, a circuit arrangement, and method for indicating the status of one of a plurality of safety switches which are connected in series between standard signal terminals of a safety relay.
Safety relays are apparatuses intended to ensure the safety of humans working in the environment of an industrial process. Safety relays are used, for example, to detect the opening of emergency stop switches or other machine lockout switches such as interlock switches guarding a gate or limit switches.
Safety relays and safety devices such as the above-mentioned switches have to be designed to meet stringent requirements defined in worldwide adopted safety standards. These standards intend to achieve high reliability. High reliability is achieved particularly by applying redundancy, diversity, and monitoring principles. In this context reference is made, for example, to a publication of Rockwell Automation: “SAFETY-BR001B-EN-P”, April 2002, pages, 10 to 13.
Safety relays provide, for example, internal checking of fault conditions such as jammed, welded, or stuck contacts of safety switches. Moreover, safety switches such as limit switches, which already have redundant normally closed safety contacts for use with dual-channel safety relays, are additionally provided with an auxiliary contact for status indication.
Safety relays offer the possibility to connect two or more safety switches in series with an input of a safety relay. When checking the proper operation of the switches it is, for example, in the case of welded contacts of one of the switches, difficult to find out which one of the serially connected switches failed to open. There is the possibility to visually inspect each one of the switches. However, this is cumbersome and time consuming. Another possibility is to provide each one of the switches with auxiliary contacts for status inspection. However, this is rather expensive and requires additional wiring.